Unsaturated polyester resins are well-known and useful materials for making molding compositions with a broad application in the manufacture of automotive and other products. One of the main deficiencies of cured unsaturated polyesters and fiber reinforced polyesters (FRP's) made from them is the inherent brittleness of the polymer matrix. Cracking at room and elevated temperatures is a principal cause of failure in production and in service of the molded parts.
Improvements in crack resistance in resin materials for SMC's have been reported by the incorporation of flexible ethers (diethylene glycol) or acids (adipic acid) in the unsaturated polyester. This approach using the so-called flexible polyesters has been only moderately successful, since the modulus and high temperature properties of the molded composite significantly decline with increasing amounts of these materials, and frequently they are faulted as sacrificing too much in their mechanical properties and dimensional stability.
Further, rigid unsaturated polyesters which contain a high fumarate content have traditionally given the best surface quality when used with common low profile additives to control shrinkage. Consequently, they have been the materials of choice for Class A automotive panels. As well as showing a great loss of mechanical properties, flexible polyesters sacrifice surface quality even when used with common LPA's. This has discouraged the use of such flexible unsaturated polyesters for vehicle panels.
A number of strategies for improving the toughness and crack resistance of polymeric materials have been demonstrated in thermoset epoxies. The most common and successful technique is the creation of a second elastomeric phase through a phase separation process during cure. This process has found only limited success in unsaturated polyesters for several reasons. The reactive liquid rubbers used to make the elastomer phase contribute to an undesirable high viscosity, the rubber phase separates before curing, and the reactive liquid rubbers contribute to higher shrinkage resulting in poor surface quality of the molded part.